As a tubular structural member incorporated in construction equipment as a part of a work implement, there have been widely used hollow rectangular cross-section members made up of four plate materials, such as hydraulic excavator booms, deep excavation system arms and mobile crane booms. Such a rectangular cross-section structure is the most popular for these members because it provides high strength for the members to stand harsh working environment.
However, the typical booms and arms of a rectangular cross-section tend to be heavy in weight as a whole. If a work implement having such a heavy long member as a chief component is mounted on the front part of a travelling vehicle, the vehicle will loose a balance of total weight. Therefore, there arises a need for countermeasures such as, for example, use of a heavy counterweight and elongation of the rear end of the vehicle. In addition, the rectangular cross-section members have revealed the disadvantage that the total weight of the vehicle equipped with the work implement increases, accompanied with many problems such as increased fuel cost.
With the intention of reducing the weight of a work implement provided with a tubular structural member, many attempts have been made to provide triangular cross-section tubular structural members which are more likely to achieve weight reduction compared to the conventional rectangular cross-section tubular members. For example, Japanese Patent Publication (KOKOKU) Gazette No. 3-19329 discloses a boom of an inverted trapezoidal shape for a hydraulic excavator, which is tapered down toward the lower face. Japanese Patent Publication (KOKAI) Gazette No. 2000-51932 discloses a method of producing a triangular tube for use in a construction vehicle. The triangular tube is produced in such a way that one plate is bent and but-welded in one place and then, each corner is formed into a circular arc shape, thereby increasing the quality of appearance to achieve high precision.
U.S. Pat. Publication No. 4,728,249 discloses a telescoping boom formed by assembling triangular cross-section members. As shown in FIGS. 6(a) and 6(b), this telescoping boom 100 having a triangular cross-section structure has a hexagonal cross-section in the strict sense and its three sides 101 are shorter than the other three sides 102. The panels corresponding to these sides are connected by welding their respective longitudinal side edges. The inner boom section nested in the outer boom section is supported and guided by rollers (not shown) mounted to the outer boom section.
As described above, various members of tubular structure for use in work implements have been devised heretofore in the prior art, but there are still many problems to be solved in achieving rigid, compact structures such as booms having the telescopic function. Specifically, the boom disclosed in U.S. Pat. No. 4,728,249, in which a triangular sectional configuration (substantially hexagonal sectional configuration) is employed instead of the known rectangular sectional configuration, has accomplished the object of weight reduction but imposed the problem of an increased number of welded places which problem leads to decreased processing accuracy, to say nothing of increased manufacturing cost. In addition, if processing accuracy decreases, outside dimension larger than necessary is required because a large gap has to be provided between the outer tube and the member telescopically received in the outer tube. Therefore, the inner tube (i.e., telescopically moving boom) is structured to be guided by guide rollers mounted on the outer boom section. This arrangement is undesirable in view of weight reduction.
The triangular tube production method disclosed in Japanese Patent Publication Gazette No. 2000-51932 is advantageous in manufacturing a tubular structural member of a triangular sectional configuration but should be further contemplated as to what kind of structure needs to be employed for producing a tubular structural member having a telescopic function, that is, a slide arm, by use of the technique disclosed in the above publication.
The structure disclosed in Japanese Patent Publication Gazette No. 3-19329 is associated with a single member having an inverted trapezoidal sectional configuration. Like the above-described boom requiring a welding process, this also presents outstanding problems in the production of a slide arm which is an object of the invention.
The present invention has been made in such a background and an object of the invention is therefore to provide a slide arm for a work implement which has a simplified structure as well as increased assembling precision and therefore can perform smooth operation.